The present invention relates to an optical disk device, and particularly to an optical head of the optical disk device making use of holograms and an optical information reading method applied in the optical head.
Along with progress of high-density and large-capacity technology of memory devices, narrower tracks and finer pits are pursued in optical disks. For enabling the finer pit size, technical improvement of the optical head performance such as converging performance for irradiating a micro-spot, fine focusing control, accurate tracking servo or high sensitivity to the pit signal is indispensable. For realizing the high performance and the miniaturization as well of the optical head, application of the hologram to the optical head has been earnestly studied.
FIG. 6 is a perspective view schematically illustrating an optical head making use of a hologram which is described in "Optical Disk System for Multimedia", by Imanaka et al., National Technical Report, Vol. 40, No. 6, pp. 771-778, December 1994, (hereinafter called the first prior art). In the first prior art of FIG. 6, a laser light which is reflected by a recording medium 602 and made parallel through an object lens 605, is divided into two beams by a two-portion grid-type hologram 606 and converged by a collimate lens 604. The converged two beams of +1.sup.st order waves are detected by a pair of photo-detectors 607 to be processed by a signal processor. This is a typical example of a CFT (Correct Far-field Tracking) hologram head.
FIG. 7 is a side view illustrating another prior art example of the hologram head, which is disclosed in a Japanese patent application laid open as a Provisional Publication No. 248144/'87 (hereinafter called the second prior art). For miniaturizing and lightening optical pickup-mechanism, the collimate lens and the object lens are replaced with phase-type holograms, whereby an incident laser light is diffracted and converged on a recording medium, in the second prior art.
Referring to FIG. 7, a laser diode 703 generates a laser light S-polarized having an oscillation plane perpendicular to the drawing sheet, which penetrates a first phase-type hologram 706b being incident with a non-Bragg angle, and diffracted by a second phase-type hologram 706a having interference fringes perpendicular to the sheet to be converged on an optical disk 702. Between the second phase type hologram 706a and the optical disk 702, there is provided a quarter-wavelength plate 705, which makes circularly-polarized the laser light to be converged on the optical disk 702, and makes P-polarized the laser light reflected by the optical disk 702 to have an oscillation plane parallel to the sheet, so that the reflected laser light penetrates the second phase-type hologram 706a and is diffracted by the first phase-type hologram 706b being incident with a Bragg angle to be converged on a photo-detector 707.
In order to miniaturizing the optical head, the size of its optical system, that is, the light-path length between the light source or the photo-detector and the recording medium should be reduced. However, the reduction of the light-path length is limited in the first prior art of FIG. 6, because the collimate lens and the object lens inserting the hologram between them should be arranged at a certain distance to the light source and the photo-detector adjoining with each other.
For miniaturizing and thinning the optical system, the collimate lens and the object lens are replaced with holograms in the second prior art of FIG. 7. However, the second prior art requires high precision phase-type holograms minutely arranged, considering complex polarization components of the holograms whereon the laser light is incident with the Bragg angle. Further, the complexity of the diffraction efficiency may cause output instability of the optical head.
Still further, the servomechanism of the second prior art depends on photo-detection of a single reflection beam. Therefore, the focusing and tracking accuracy should be degraded compared to ordinary servomechanism wherein the reflection beam is split into two beams for improving detection accuracy.